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The grey, short-tailed opossum - a mouse-sized critter from the rainforests of South America - has become the first marsupial species to have its genome sequenced.

An international team of more than 60 researchers publish the details of the opossum's genetic make-up in the journal Nature today.

Their early findings shed light on the differences and similarities between marsupials, whose young are typically carried in pouches, and placental mammals like humans.

For example, they show that marsupials have many immune system genes that are distinct from those found in placental mammals.

They also offer clues about how we evolved the biological signals that regulate the way genes are expressed.

Scientists are particularly interested in marsupial genetics because they diverged from other mammal species so long ago, around 180 million years ago, says Professor Jenny Graves, from the Australian National University, who led the Australian part of the project.

"They're very important because they are so distantly related to humans," she says.

Comparing the human genome with our distant evolutionary cousins helps reveal which of our genes are old and which we have acquired relatively recently, she says.

Also this year, scientists are planning to publish a genome sequence for the platypus, a member of the monotreme group of mammals, which are thought to have branched off from the other mammal species even earlier than the marsupials.